The present invention relates to a magnetically anisotropic R-T-B magnet based on a transition metal (T), a rare earth element (R) including Y and boron (B), and more particularly to a magnetically anisotropic magnet showing a maximum energy product distribution substantially uniform between its center portion and its circumferential portion so that it can be suitably used for voice coil motors, magnetrons, linear motors, MRI, etc.
With respect to permanent magnets used in magnetic circuits of voice coil motors, magnetrons, linear motors, MRI, etc., it is important not only that they have large absolute values of maximum energy product (BH).sub.max, but also that the maximum energy product is distributed substantially uniformly between the center portion and the circumferential portion. Particularly recently, permanent magnets having such properties are increasingly needed.
Since permanent magnets based on rare earth elements (R), transition metals (T) and boron (B) (hereinafter referred to as "R-T-B magnets") are inexpensive and show high magnetic properties, they have been attracting much attention as those satisfying the above-mentioned requirements (Japanese Patent Laid-Open No. 61-266056).
The R-T-B magnets are classified into sintered magnets and rapidly quenched magnets. Among them, permanent magnets produced by rapidly quenching alloy melts to form thin ribbons or flakes, finely pulverizing them, hot-pressing the pulverized alloys and then subjecting them to high-temperature plastic working to impart magnetic anisotropy thereto (hereinafter referred to as "plastically hot-worked magnets") have been increasingly attracting attention (Japanese Patent Laid-Open No. 60-100402).
Known as such plastically hot-worked magnets are those showing maximum energy product satisfying the relation: (A-B).times.100/A.ltoreq.4, wherein A represents maximum energy product in a center portion and B represents maximum energy product in a circumferential portion, an average value of the overall maximum energy product being 20 MGOe or more with little unevenness (Japanese Patent Laid-Open No. 1-251703).
However, as is clear from the above relation, these plastically hot-worked magnets require A.gtoreq.B, and their overall maximum energy products are 22.9-25.2 MGOe (See Examples), which are insufficient to constitute high-performance magnetic circuits. The reason why A.gtoreq.B should be met is considered that the center portions of the magnets are less plastically flowable than the circumferential portions due to friction between the magnets and die surfaces in the process of plastic working. In any case, such restrictions are undesirable to meet the demands in the market place.
Incidentally, although Japanese Patent Laid-Open No. 1-251703 is silent, the uneven deformation of the magnets causes a bulge phenomenon which leads to large cracks in the circumferential portions of the resulting magnets. This is a serious problem in the case of obtaining high-performance magnets. Particularly, in the case of voice coil motors used in outside memory apparatuses of computers, dusting due to cracks causes serious troubles.